Particularly in the last decade, liquid crystals have been introduced into various industrial areas in which electro-optical and display-device properties are required (for example in watch, calculator and typewriter displays). These display devices are based on dielectric alignment effects in the nematic, cholesteric and/or smectic phases of the liquid-crystalline compounds, where--caused by the dielectric anisotropy--the molecular long axis of the compounds adopts a preferential alignment in an applied electric field. The usual response times in these display devices are too long for many other potential areas of application of liquid crystals. This disadvantage is particularly noticeable if a large number of pixels must be addressed. Production costs of equipment containing relatively large screen areas, for example of video equipment, are then generally too high.
In addition to nematic and cholesteric liquid crystals, optically active smectic liquid crystal phases have also been increasing in importance over the last few years.
Clark and Lagerwall were able to show that the use of ferroelectric liquid-crystal systems in very thin cells results in electro-optical switching or display elements which have response times faster by a factor of 1000 compared with conventional TN ("twisted nematic") cells (cf., for example, Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, October Meeting 1985, San Diego, Calif., USA). On the basis of this and other favorable properties, for example the possibility of bistable switching and the virtually viewing angle-independent contrast, FLCs are in principle highly suitable for the abovementioned areas of application, for example via matrix addressing.
For electro-optical or fully optical components, either compounds are required which form tilted or orthogonal smectic phases and are themselves optically active, or ferroelectric smectic phases can be induced by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range.
In order to achieve good contrast in electro-optical components, a uniform planar alignment of the liquid crystals is necessary. Good alignment in the S.sub.A and S*.sub.c phase can be achieved if the phase sequence of the liquid-crystal mixture is, with decreasing temperature: EQU isotropic.fwdarw.N.sup..cndot. .fwdarw.S.sub.A .fwdarw.S.sup..cndot..sub.c
The prerequisite is that the pitch of the helix in the N* phase is very large (greater than 10 .mu.m) or even better is fully compensated (see, for example, T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan; M. Murakami et al., ibid. pp. 344-347). This is achieved by adding a further optically active dope which induces a right-hand helix to the chiral liquid-crystal mixture which has, for example, a left-hand helix in the N* phase, in such amounts that the helix is just compensated.
A further prerequisite for the use of the SSFLCD effect (surface-stabilized ferroelectric liquid-crystal display) of Clark and Lagerwall for uniform planar alignment is that the pitch in the smectic C* phase is significantly greater than the thickness of the display element (Mol. Cryst. Liq. Cryst. 94 (1983), 213-233 and 114 (1984), 151-187). As in the case of the cholesteric pitch, this is achieved by using dopes having the opposite rotation of the helix.
Ferroelectric liquid-crystal displays can also be operated by utilizing the DHF (distorted helix formation) effect or the PSFLCD effect (pitch-stabilized ferroelectric liquid-crystal display, also known as SBF=short pitch bistable ferroelectric effect). The DHF effect has been described by B. I. Ostrovski in Advances in Liquid Crystal Research and Applications, Oxford/Budapest, 1980, 469 ff.; the PSFLCD effect is described in DE-A 3 920 625 and EP 0 405 346 A2. In contrast to the SSFLCD effect, utilization of these effects requires a liquid-crystalline material having a short S.sub.c pitch.
The optical response time .tau.[.mu.s] of ferroelectric liquid-crystal systems, which should be as short as possible, depends on the rotational viscosity of the system .gamma.[mPas], the spontaneous polarization P.sub.s [nC/cm.sup.2 ] and the electric field strength E[V/m], in accordance with the equation ##EQU1##
Since the field strength E is determined by the electrode separation in the electro-optical component and by the applied voltage, the ferroelectric display medium must have low viscosity and a high spontaneous polarization to achieve a short response time.
Finally, in addition to thermal, chemical and photo-chemical stability, a small optical anisotropy .DELTA.n, preferably .apprxeq.0.13, and a low positive or preferably negative dielectric anisotropy .DELTA..epsilon. are required (see S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, Oct. Meeting 1985, San Diego, Calif., USA).
The totality of these requirements can only be achieved by means of mixtures comprising a plurality of components. The base (or matrix) used preferably comprises compounds which if possible themselves already have the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.c. Further components of the mixtures are frequently added in order to reduce the melting point and to broaden the S.sub.c and usually also the N phase, to induce spontaneous polarization, for pitch compensation and to match the optical and dielectric anisotropy; however, the rotational viscosity, for example, should if possible not be increased.
Polyfluorinated phenyl derivatives have already been described a number of times as components of liquid-crystal mixtures, for example derivatives of 1,2-difluorophenylene in EP-B 357 702 and EP-A 500 210, derivatives of 1,3-difluorophenylene in DE-A 4 127 450, derivatives of 1,2,3,4-tetrafluorophenylene in DE-A 4 037 519, derivatives of 1,2,4,5-tetrafluoro-phenylene (presented at the 12th International Liquid Crystal Conference, 1988, Freiburg, Abstract FE 68, and WO 89/09203), and specific dopes containing a 1,2,4-trifluorophenylene or 1,2,4,5-tetrafluorophenylene unit in JP-A 5-17406.
Since, however, the development of ferroelectric liquid-crystal mixtures in particular can in no way be regarded as complete, the manufacturers of displays are interested in a very wide variety of components for mixtures. Another reason for this is that only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid-crystalline mixtures too.